Montador spent the majority of the 2001–02 season with Saint John where he scored 9 goals and 25 points in 67 games.[2] He earned his first recall to Calgary early in the season and made his NHL debut on November 23, 2001 – and recorded his first point with an assist – against the Buffalo Sabres.[3] Montador's first NHL goal came on January 8, 2002, against goaltender Garth Snow in a 5–2 victory over the New York Islanders, and he also played with Team Canada at the 2001 Spengler Cup.[6] He appeared in 11 games for Calgary and recorded three points.[2] In 2002–03, Montador again began the year with Saint John where, during one early-season contest, he tied an AHL record for shots in a game with 14.[7] He was recalled to Calgary days later to replace defenceman Bob Boughner, who suffered a broken thumb.[7] Montador remained with the Flames for the remainder of the season; he recorded a goal and an assist in 50 games.[2]

As a labour dispute ultimately led to the cancellation of the 2004–05 NHL season, Montador was one of many players who spent the season in Europe; He signed a deal to play with the Mulhouse Scorpions of France's Ligue Magnus and brought Flames teammate Steve Reinprecht over as well.[10] Montador appeared in 15 games with Mulhouse where he scored 1 goal and added 7 assists.[2] The Scorpions went on to win the Ligue Magnus title.[11] Montador returned to Calgary to begin the 2005–06 season after the league and players resolved their dispute, however head coach Darryl Sutter struggled to find a place in the lineup for him, and by the start of December, Montador had played only seven games. Hoping to improve his team and to give Montador a place to play regularly, Sutter and the Flames traded Montador, along with Dustin Johner, to the Florida Panthers in exchange for Kristian Huselius on December 2, 2005.[12]

Montador played full-time on the Panthers' defence; he finished the 2005–06 season with 58 games played combined between the two teams, scored 2 goals and added 5 assists. He then recorded nine points in 72 games in 2006–07 and then set personal highs in games played (73), goals (8) and points (23).[2] Following the season, Montador left the Panthers and returned to the west as he signed a one-year contract with the Anaheim Ducks.[13] His stay in Anaheim was short lived as, after recording 20 points in 65 games for the team, the Ducks traded Montador on March 4, 2009, to the Boston Bruins in exchange for Petteri Nokelainen.[14] He finished the regular season with an assist in 13 games, then added four points in 11 playoff games.[2]

Again a free agent, Montador signed a two-year, $3.1 million contract with the Buffalo Sabres. The team hoped that he could add a physical presence to their team after losing Jaroslav Špaček to the Montreal Canadiens.[15] Montador recorded 23 points in 78 games for Buffalo in 2009–10 then posted the highest totals of his career of 21 assists and 26 points, in 73 games, in 2010–11.[2] However, his play declined late in the season, and he was ultimately left out of the lineup as a healthy scratch for Buffalo's deciding game seven (a loss) to the Philadelphia Flyers; Consequently, the team opted not to re-sign Montador and traded his rights to the Chicago Blackhawks on June 28, 2011, for a seventh round selection in the 2011 NHL Entry Draft.[16] Chicago immediately signed him to a four-year, $11 million contract.[17]

Montador appeared in 52 games for the Blackhawks in 2011–12, scored five goals and added nine assists. However his season was ended after he suffered a serious concussion during a game. He required almost a full year to recover from the injury, and as a result played only 14 games for Chicago's AHL affiliate, the Rockford IceHogs late in the 2012–13 season before Chicago opted to buy-out the remaining two years of his contract.[18] Montador left the NHL to join Croation team KHL Medveščak Zagreb of Russia's Kontinental Hockey League (KHL). He appeared in 11 games for Zagreb and recorded three assists in what was the final season of his career.[2]

Montador spoke openly of the depression he suffered following his concussion issues in Chicago,[20] and according to his brother Chris, had spent a great deal of time researching the effects of head injuries. Chris noted that both he and his brother had suffered concussions as children, and that Steve had promised to donate his brain to medical researchers studying brain issues.[21] On February 15, 2015, Montador was found dead in his Mississauga home.[18] According to his brother, "he just either stopped breathing or his heart went".[21] Four days after his death, Montador's girlfriend gave birth to to their son.[22]

Montador's death at age 35 was a shock to players throughout the NHL; many of his former teammates spoke positively of his influence and attitude.[19]Daniel Carcillo reflected on his friendship with Montador in an interview for The Players' Tribune and cited Montador as a positive influence who helped him overcome his substance abuse problems. Carcillo noted that he witnessed Montador's mental state slowly deteriorate before his death and called upon the NHL community to establish a formal exit-procedure for players suffering from post-concussion syndrome.[23][24] Chris Montador had also noted a change in Steve's behaviour prior to his death.[21]

Researchers studying Montador's brain noted that he had suffered from chronic traumatic encephalopathy (CTE), a degenerative brain condition that doctors have noted is caused by concussions. As a consequence of the revelation, lawyers representing his estate stated their intention to file a lawsuit against the NHL.[25]